The present invention relates to an organic electroluminescent device that can be applied to a flat panel display or illumination light source. More particularly, the present invention relates to an organic electroluminescent device that operates at a low driving voltage and has low power consumption.
Research towards practical use of organic electroluminescent devices (hereinafter also referred to as organic EL devices) has been conducted actively in recent years. Since organic EL devices are able to realize high current density at low voltage, they are expected to be able to realize high emission luminance and luminous efficiency. These organic EL devices are provided with a first electrode and a second electrode surrounding an organic EL layer, and the electrode on the side on which light is extracted is required to have high transmittance. Normally transparent conductive oxide (TCO) materials (such as indium tin oxide (ITO), indium zinc oxide (IZO) or indium-tungsten oxide (IWO)) are used for these electrode materials. Since these materials have a comparatively large work function of up to 5 eV, they are used as hole injection electrodes (anodes) of organic materials.
Light emitted by organic EL devices is obtained as a result of light being released accompanying relaxation of the excitation energy of excitons generated by holes injected into the highest occupied molecular orbital (HOMO, generally measured as ionization potential) of a light-emitting layer material, and electrons injected into the lowest unoccupied molecular orbital (LUMO, generally measured as electron affinity). In general, organic EL devices employ a laminated structure having a light-emitting layer in addition to all or a portion of a hole injection layer, a hole transport layer, an electron transport layer and an electron injection layer in order to efficiently carry out hole injection and electron injection into the light-emitting layer.
In recent years, methods for lowering the voltage and power consumption of organic EL devices employing this type of laminated structure for organic EL devices have attempted to improve effective charge mobility and lower the charge injection barrier from an electrode by doping charge transport layers other than the light-emitting layer with impurities, and a technology has been disclosed for lowering the driving voltage of these devices (Japanese Patent Application Laid-open No. H4-297076; Japanese Patent Application Laid-open No. H11-251067; Japanese Translation of PCT Application No. 2004-514257; Organic Electronics, Vol. 4, Issues 2-3 (September 2003), p. 89; Applied Physics Letters, Vol. 73, Issue 20 (November 1998), p. 2866).
This technology is similar to the p-type doping and n-type doping technology employed in inorganic semiconductors. For example, in the case of a hole injection layer or hole transport layer, by mixing an impurity in the form of a material having high electron acceptability (acceptor) into the material of the hole transport layer that composes them (to also be referred to as a hole transport material in the present description), the hole injection barrier from an electrode (difference between the work function of the anode and the HOMO level of the adjacent hole transport material) can be lowered, or the effective carrier mobility of the hole transport layer can be improved. In the case of an electron injection layer or electron transport layer, by mixing an impurity in the form of a material having a high electron-donating ability (donor) into the material of the electron transport layer (also referred to as an electron transport material in the present description), the electron injection barrier from an electrode (difference between the work function of the cathode and the LUMO level of the adjacent electron transport material) can be lowered, or the effective mobility of the electron transport material can be improved.
On the other hand, phenanthroline derivatives and silole derivatives have been reported to be electron transport materials having even higher electron mobility (See, for example, Applied Physics Letters, Vol. 76, Issue 2 (January 2000), p. 197; Applied Physics Letters, Vol. 80, Issue 2 (January 2002), p. 189), and by using these materials in an electron transport layer, driving voltage can be reduced considerably for electron injection layers and electron transport layers without having to use n-type doping techniques.
However, as a result of promoting injection of electrons into a light-emitting layer by using an electron transport material like that described above, the hole-electron recombination region ends up being extremely biased toward the anode side, namely the hole transport layer/light-emitting layer interface, and although this allows current to flow at a lower voltage, there are cases in which luminous efficiency decreases remarkably. This phenomenon is particularly remarkable when the host of the light-emitting layer does not have a dominantly high hole transport ability in terms of its charge transport properties.
In cases in which the supply of electrons to the light-emitting layer has become excessive in this manner, by using a hole transport material having electron affinity sufficiently smaller than (0.15 eV or more) the electron affinity of the light-emitting layer for the hole transport layer in contact with the light-emitting layer, electrons are blocked at the hole transport layer/light-emitting layer interface, and as a result of being trapped within the light-emitting layer, improve luminous efficiency. This technology is disclosed in Japanese Patent Application Laid-open No. 2005-26210.
In addition, a technology for preventing deterioration of luminous efficiency or emission lifetime is disclosed in Japanese Patent Application Laid-open No. 2006-66872 that prevents excessive supply of electrons to a light-emitting layer by providing an electron limiting layer that limits the movement of electrons together with an electron transport layer that promotes transport of electrons between the light-emitting layer and the cathode.
In addition, Japanese Patent Application Laid-open No. H6-314594 discloses a technology for providing an organic EL device that lowers device driving voltage and has superior durability by providing a plurality of carrier injection layers.
The technology disclosed in Japanese Patent Application Laid-open No. 2005-26210 was unable to sufficiently prevent a decrease in luminous efficiency of the device while also impairing emission lifetime. This is thought to be due to deterioration of the hole transport material as a result of numerous electrons reaching the hole transport layer/light-emitting layer interface.
Although use of the technology disclosed in Japanese Patent Application Laid-open No. 2006-66872 certainly makes it possible to prevent deterioration of luminous efficiency or emission lifetime, as a result of limiting movement of electrons, the effect of reducing voltage is diminished, thereby preventing the inherent effect of reducing voltage from being adequately obtained.